Lately, a vehicle seat using a three dimensionally structured net material (three-dimensional knitted fabric) which is of a thin-type, but able to exhibit a high cushioning performance, and which has a large number of cylinders and is accordingly able to exhibit excellent air permeability, has been known. The three-dimensional knitted fabric has a truss structure (three-dimensional structure) which does not easily elastically wear out. The truss structure is formed by using a large number of connecting threads to join a pair of ground knit fabrics which are disposed so as to be separated from one another. The three-dimensional knitted fabric has excellent air permeability, body pressure distribution performance, and impact absorbing performance, and in spite of being of a thin type, can impart characteristics similar to those obtained by a high elastic polyurethane foam generally used as a cushion material.
However, when a cushion material having high stiffness is used for a vehicle seat or the like, a load from a human body is generally concentrated on a muscle portion in the vicinity of a tuber ischiadicum. This creates an uncomfortable sensation, and even causes numbness when an occupant is seated on a vehicle seat for a long period of time.
On the other hand, it is known that ranges of vibration which largely influence riding comfort are around 5 Hz at which range a skeleton of an occupant (a human body) swings vertically and at 2 Hz or less at which range a human body swings in a forward-backward (longitudinal) direction. Therefore, it is considered that an ideal cushion material should have characteristics whose tolerable vibration peak is outside the aforementioned vibration ranges, and in addition, whose vibration-transmitting ratio is relatively low, within a range of 6 to 8 Hz, and accordingly, has resonance with the internal organs of an occupant.
In view of the aforementioned facts, an object of the present invention is to provide a seat using a three-dimensional knitted fabric of a thin and light type and having superior characteristics as a cushion material as described above, in which riding comfort can be improved by alleviating concentration of a load on a tuber ishiadicum of an occupant, reducing occurrences of numbness when an occupant is seated for a long period of time, and compared with the past, alleviating vibration-transmitting characteristics caused by the impact of vibration on a human body (seated occupant).